Pulse modulation communication systems



May 25, 1948.. P. K. CHATTERJEA ETAL. 2,441,954

-PULSE MODULATION COMMUNICATION SYSTEMS i y 1945 2 Sheets-Sheet i I nventof EancuunKmKCnMER Mamas Kntuxk 3mm May 25, 1948. P. K. CHATTERJEA El'AL 2,441,954

PULSE MODULATION COMMUNICATION SYSTEMS Filed May 21, 1945 2 Sheets-Sheet 2 v In ventor Shawna Kmn qmmwazn d nib uunuk EGEi y I Aliozney Patented May 25, 1948 PULSE MODULATION COMIWUNICATION SYSTEMS Prafulla Kumar Chatterjea and James Kinloch Beney, London, England, assignors to Standard Telephones and Cables Limited, London, England, a British company Application May 21, 1945, Serial No. 594,871 In Great Britain December 20, 1944 Claims. (Cl. 179171.5)

The present invention relates to arrangements for generating rectangular electrical pulses of adjustable, duration.

In the specification of co-pending British application No. 5940/44 there is described an arrangement involving the use of a gas-filled triode or soft valve for generating pulses of adjustable duration. The present specification describes an alternative arrangement employing only ordinary hard valves. The principal feature of the invention consists in an arrangement of two ordinary triode valves coupled together in such a manner that they exhibit properties very similar to the firing and extinguishing characteristics of the well-known gas-filled triode.

The invention provides an arrangement for generating periodic electrical waves comprising two thermionic valves each having at least a cathode, a control grid and an anode, a source of anode current for the said valves, meansrfor coupling the anode of the first valve to the control grid of the second valve, means for applying saw-tooth waves to the control grid of the first valve, means for feeding back the output variations of the second valve in positive phase to the input of the first valve, andmeans for deriving periodic waves from either of the said anodes.

According to a minor modification, the two valves are replaced by a double triode having a single cathode common to both triode portions.

The invention will be described with reference to the accompanying drawings, in which- Fig. 1 shows a schematic circuit diagram of an arrangement according to the invention;

Fig. 2 shows a valve characteristic; and

Fig. 3 shows wave form curves employed to explain the action of Fig. 1.

Referring to Fig. 1, which shows a schematic circuit diagram of one embodiment of the invention, a double triode vacuum valve i has a cathode 2 common to both sections. The two separate control grids are designated 3 and 4, and the corresponding separate anodes are 5 and 6, respectively. Alternatively,two similar single triode valves in separate envelopes, with their cathodes connected together, couldbe used instead if preferred. The cathode 2 is connected to the grounded negative terminal "l of the high tension source through a load resistance 8. The two anodes 5 and 6 are connected to the positive high tension terminal 9 through individual anode resistanceslfl and H, and a common resistance 12, a bypasscondenser l3, being provided.

. An arrangement for biassing the control grids 3 and 4 either positively or negatively comprises two equal resistances l4 and [5 connected in series across a direct current source intended to be connected to the terminals l6 and IT. The junction point of these resistances is connected to ground, and the terminals 16 and I1 arefurther shunted by two potentiometers l8 and IS.- The control grids 3 and 4 are respectively con-J nected to the movable contacts of potentiometers l8 and I9 through resistances 20 and 2|, the usual by-pass condensers 22 and 23 being provided.

The anode 5 is connected to the control grid 4- by a blocking condenser 24. A saw-tooth wave generator (not shown) of any suitable typeis intended to be connected to the input terminals 25 and 26, and supplies waves through a resistance 21 to the control grid 3. This resistance forms a reducing potentiometer with the resistance 20, so that a suitable .iraction of the sawtooth voltage is applied to the control grid 3. The resistance 21 is shunted by a-condenser 28, which compensates, in known manner for the frequency distortion introduced by the stray capacity shunting the resistance 20. a

The anode 6 is connected to the output terminal 29 through a condenser 30. A'resistance 31 is provided to shunt the terminals 1 and 29, andforms with the condenser 30 a pulse differentiating circuit. Modulating voltages maybe applied to the control grid 3 from the input terminal 32 through the blocking condenser 33 and resistance 34; or alternatively, they may be applied to the other control grid 4 from the input terminal 35 through the blocking condenser 36 and resistance 31.

The manner in which the circuit is believed to operate will be explained with reference to the characteristic curve of Fig. 2, and to-wave form diagrams of Fig. 3 which are based oncurves obtained experimentally with a circuit in which the significant elements had the values quotedin the table below:

Capacity,

Element Mfds.

Element gig g The two triode portions of the valve l'ares'up posed to be similar, a typical characteristic curve for either being shown in Fig. 2. This shows the relation between the anode current Ia and the control grid voltage Vg with respect to the cathode. The grid bias of the left hand portion of the valve is adjusted by means of the potentiometer I8 to a value such that whena driving voltage is applied: at the terminal 25, the anode; current Ia varies between a value given by the point 33 on the lower band, a little above the cut-oil point; e g and a value given by a point 39 somewhere below the saturation point of the valve, so that the worlring point varies over the portion of the charac'- teristic curve between the points-38-and"39, and a the grid voltage swings between the correspondj ing values OM and ON. The grid bias of the right hand half .of the valve is adjusted by the potentiometer Hi to a relatively large positive value Q, when the left handhalfgisinxthe con+- dition represented by the point 38'; This value feedbackithrou'ghthe cathode resistance 8 acting to accelerate the change.

The. corresponding changes in the cathode voltage are show'n in the curve D. Thus during the: periods of thejanode pulses the cathode voltage isfir'elatively'. large and rises slightly as the 0Q should correspond to a point on the upper bend of the characteristic curve, or it may be well above the saturation voltage for the valve.

Sinceboth anode currents. flow throughthe resistance 8, both'of these-currents afiect the potentials of both grids with respect to the common cathode. and the biasqadjustmentsmentioned above: should be suchas to'produce the desired values-OMandOQ simultaneously;

'The total grid voltage'swingof the right-hand half: of the'valveisPQ; whichis' greater than the corresponding swing MN'for the other half of the valve"; The voltage corresponding'to Pshould be well beyond thei cut-ofi'i voltage. Thus theanode current of theright-handhalf of the-valve'alternates betweenarelativelylarge or saturatedvalve, andzero. Thesepoints 'will be made clearer with reference to the wave form curves of Fig. 3. Curve" A- shows: the: 's'alw tooth' wave voltage applied between 'the"t'erminals"25' and 26' ofFig. .1 with respect to time, thedotted line. 40 representing zero voltage. Curve B'represents the corresponding-voltage-variations of the anode' the dotted line 41 representing the voltageat the junction'point of resistanceslll an'd' H. Curve C represents the voltage variations of the anode 5,

the dotted line 4| representing the same as behand' half of the valveis small, so-that" the potential of the anode" 5 will be a little below the voltage of the dotted line 41, as represented by thepoints 43. The anode current in the'righthand half of .the valve is a maximum and the corresponding anode voltage is' a minimum as indicated by the points 44'. .As the' saw-tooth voltage'increases from zero, the anode current of the left-hand half increases and theivoltageof the corresponding anode falls. This change will, however, be small due to the action of the cathode resistance 8. This fall of voltage is communicated 'throughth'e condenser 24 tothe control grid 4, and nothing further happens until an appreciable :decrease in the anode current of the right-hand half of the "valve occurs. This decrease acting in the'resistance 8' increases thepotential of the control grid 3 with respect to the cathode, and so accelerates the fall of voltage of the anode 5, and-this change-is fedround again' so-that-ineffect the right-hand half proceeds rapidlybeyondr the cut-off; the voltage of the anode suddenly rising to the maximum representediby. the1points '4'5. Thesudd'en decrease in cathode potential" produces a sudden increase "in the left-hand anode current and a corresponding left hand anode current rises. With the cutting off of therightehand half, there is a sudden fall in the cathode voltage followed by an immediate smallfrise "and then a slower risecorresponding to the slow change in anode voltage, between the points 48 and 41. Whenthefly-back strokeoccurs, thereis'a relatively rapid fall in cathode voltagefollowed by a sudden rise atthemoment of saturationof the right-hand portion;of the valve.

It will be seenthat the voltage pulsesl curve C) which can be obtained from the anode 6 are very nearly rectangular. The pulses (curve B)- could, if desired; be obtained-from the anode 5, but they are of inferior form due to thefact that the lefthand half of the valveisonly operatedover part of its characteristic curve. 7 V

Theitimingi of theleading edges of the pulses from the anode Bis fixed by the fly-back strokes, but the timing of the traiIing edgBs can be adjusted by adjustment-of eitherof the potentiometers l8 or' l9. Reducing the negative: bias OM or the positive bias OQmakes the trailing edges occur earlier, andiricreasing eitherbias makes them ioccur later-. Thus 'by applying equal.

modulating signals at terminal 32ior at terminal 35, the'trailing'edges of the pulses from the anode 6 may be time modulated. Thus time duration modulated pulses may be obtained} direct from the anode 6, or pairs of short marking'pulse's one of which in each'pair in time-phase-mo'dulated may be'obtained from terminaLZQafterdifferentiation by theelements 30 and 3!.

It will be appreciated that the doubletrio'de l with its associated circuits operates practically in the same way as a gridcont'rolled gas filled triode of which the; cathode, control grid and anode are represented by theelements. 2; 3 and 6' respectively.

The arrangement-of Fig. l can be regarded as a two-stage amplifier in' which the cathode resistance 8 acts to feed back the output variations to the input in positivephase- It also acts to introduce localnegativefeedback in each stage separately. It will be understood that the desired results according to the invention can be obtained by introducing the positive feedb ways. 7 V

The numerical values given above for illustration are of course not'the'only values which can be used, and donot indicate any essential relations between the various elements; for example, the resistances l ll H and 12 are not'necessarily What is emailed is;

1.;An arrangement for generating variable durationpulses in accordance with-applied signal energy comprising, a double valve having two sections eachincluding'an anode and a control grid,

and a cathode'common to both sections, means for applying a direct current potential between ack in other.

' knee of its characteristic curve, means for biasing the second section to operate from below cut-off to at least the upper knee of its characteristic 1 curve, means for applying control waves tov the control grid of the first section, means for applying the signal energy to one of said control grids, and means for deriving variable duration pulses from one of said anodes.

2. An arrangement according to claim 1 wherein the means for applying control waves to the grid of the first section includes means for pro- 1 ducing and applying sawtooth waves.

3. An arrangement according to claim 1 further including means for difierentiating the variable duration pulses to produce variable time displaced pulses.

6 4. An arrangement according to claim 1 further including means for adjusting the durations of the derived pulses.

5. An arrangement according to claim 1 fur- 5 ther including means for adjusting the durations of the derived pulses including means for adjusting the bias of one of said control grids.

PRAFULLA 'KUMAR CHATTERJEA. JAMES KINLOCH BENEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,061,734 Kell Nov. 24, 1936 2,086,918 Luck July 13, 1937 20 2,211,750 Humby et al. Aug. 20, 1940 2,338,395 Bartelink Jan. 4, 1944 2338.512 Harmon Jan. 4, 1944 

